The present invention relates to a device for detecting an air/fuel ratio of a fuel mixture by probing exhaust gas resulting from combustion of the fuel mixture, and more particularly to a device for detecting an air/fuel ratio of a fuel mixture combusted in an internal combustion engine. The present invention relates also to a system, employing an air/fuel ratio detecting device, for controlling an air/fuel ratio of a fuel mixture combusted in an engine.
Up until now, numerous types of exhaust gas sensors or oxygen sensors have been developed for the purpose of measuring the oxygen concentration within the exhaust gas resulting from combustion of a fuel mixture within an engine so as to detect the air/fuel ratio of the fuel mixture.
Laid-open Japanese patent application 56-89051 discloses an oxygen sensor. This oxygen sensor is illustrated in FIG. 1 for the ease of explanation. Referring to FIG. 1, this known oxygen sensor 1 operates on the principle of an oxygen concentration cell that produces an electromotive force in response to the ratio of the oxygen concentration on one side of a solid electrolyte to that on the opposite side thereof. It comprises a base 2 of alumina, a reference electrode 3 on the base 2, an oxygen ion-conductive solid electrolyte 4 which cooperates with the base 2 to enclose the reference electrode 3, a measurement electrode 5 interposing the solid electrolyte 4 in cooperation with the reference electrode 3. The above listed elements are covered by a protection layer of porous material. For activating the solid electrolyte, a heater 7 is embedded in the base 2. With the measurement electrode 5 exposed to the exhaust gas, an electric current Ip is supplied to the oxygen sensor 1 so as to cause migration of oxygen ion through the solid electrolyte 4, resulting in generation of a reference oxygen partial pressure Pa on the reference electrode 3 and an oxygen partial pressure Pb of the exhaust gas on the measurement electrode 5. Generation of the oxygen partial pressures Pa and Pb causes an electromotive force E to be generated which may be expressed by the following Nernst's equation as: EQU E=(RT/4F).multidot.ln (Pa/Pb) (1)
where:
R=gas constant PA1 T=absolute temperature PA1 F=Faraday constant
With the same intensity of the electric current Is, the electromotive force E changes in a step manner at a predetermined air/fuel ratio. The air/fuel ratio where the E changes abruptly varies with variation in the intensity of the electric current Is.
An air/fuel ratio control system using the oxygen sensor 1 described above is known by the above mentioned laid-open Japanese patent application. According to this known air/fuel ratio control system, the intensity of the electric current Is is varied so that the rapid change in the electromotive force E takes place at a target air/fuel ratio. The electromotive force E is taken out as a sensor output voltage Vs. Since the characteristic of the sensor output voltage Vs is such that it remains constant irrespective of variation in air/fuel ratio after it has deviated by a small amount from a target air/fuel ratio, it is impossible to control the speed at which the actual air/fuel ratio is converged to the target one because the deviation cannot be determined by the sensor output voltage Vs having the above characteristic. Thus, there is the limitation to increasing the precision and response in detecting and controlling air/fuel ratio.